Today's semiconductor devices, whether discrete power or logic ICs, are smaller, run faster, do more and generate more heat. Some desktop microprocessors dissipate power levels in the range of 50 to 100 watts. These power levels require thermal management techniques involving large capacity heat sinks, good air flow and careful management of thermal interface resistances. A well designed thermal management program will keep operating temperatures within acceptable limits in order to optimize device performance and reliability.
Semiconductor devices are kept within their operating temperature limits by transferring junction-generated waste heat to the ambient environment, such as the surrounding room air. This is best accomplished by attaching a heat sink to the semiconductor package surface, thus increasing the heat transfer between the hot case and the cooling air. A heat sink is selected to provide optimum thermal performance. Once the correct heat sink has been selected, it must be carefully joined to the semiconductor package to ensure efficient heat transfer through this newly formed thermal interface.
Thermal materials have been used to join a semiconductor package and a heat sink, and to dissipate the heat from semiconductor devices, such as microprocessors. Thermal interface material (TIM) typically includes a polymer matrix and a thermally conductive filler. The TIM technologies used for electronic packages encompass several classes of materials such as epoxies, greases, gels and phase change materials.
Metal filled epoxies commonly are highly conductive materials that thermally cure into highly crosslinked materials. However, they have significant integration issues with other components of the package. For example, many times metal filled epoxies exhibit localized phase separation within the material that can result in high contact resistance. Furthermore, the metal filled epoxies can also delaminate at the interfaces.
Thermal greases are in a class of materials that offers several advantages compared to other classes of materials, including good wetting and ability to conform to the interfaces, no post-dispense processing, and high bulk thermal conductivity. Greases provide excellent performance in a variety of packages; however, greases cannot be used universally with all packages due to degradation of thermal performance during temperature cycling. It is observed that in some packages greases migrate out from between the interfaces under cyclical stresses encountered during temperature cycling. This phenomenon is known as “pump out.”
In some applications an integral heat spreader is used to spread the heat and protect the semiconductor device beneath the integral heat spreader. In some applications, air is trapped within the integral heat spreader. Air is a poor thermal conductor.
The description set out herein illustrates the various embodiments of the invention, and such description is not intended to be construed as limiting in any manner.